1. Field of the Invention
The present invention relates to an apparatus and method for detecting a heartbeat, and more particularly, to an apparatus and method for detecting a heartbeat by using a photoplethysmography (PPG) which is capable of detecting the heartbeat even under motion of a user, i.e. is hardly influenced by any motion artifact.
2. Description of the Prior Art
In general, a PPG is a method of extracting information related to a heartbeat by using a predetermined number of LEDs and optical sensors. According to the method of detecting the heartbeat using the PPG, the heartbeat can be detected through a simple sensor which is attached to only one contact point of a human body. Thus, the PPG-based method is more convenient to a user than an electrocardiogram (ECG) method in which at least two electrodes should be used, and it can be much more efficiently applied to a medical or non-medical purpose instrument. On the other hand, the PPG-based method has a disadvantage in that a large amplitude of the motion artifact is induced into a PPG signal even under any slight motion of the user. Such a motion artifact can be defined as a noise signal generated by a slight motion such as a minute quiver of the human body upon detection of the heartbeat. Therefore, in order to obtain a highly reliable heartbeat measurement, the motion artifact should be eliminated from the resultant PPG signal.
Heartbeat detection has been widely used in various fields for medical or non-medical purposes. As a typical method for detecting the heartbeat, there has been provided a method (electrocardiogram) of measuring and recording a value of an active current generated due to contraction and/or relaxation of a heart muscle by using electrodes attached onto predetermined parts of a human body.
However, the conventional heartbeat detection apparatus has problems in that the user is reluctant to use the apparatus since the electrodes are attached to or bands are bound around the human body, and in that it is very inconvenient for the user to use the apparatus unless the user has considerable knowledge of its use.
In order to solve these problems, there has been proposed a circulatory function diagnostic device disclosed in Japanese Patent Laid-Open Publication No. 2000-217796 (date of publication: Aug. 8, 2000), which can easily diagnose a function of the blood circulatory system using a waveform pattern of an acceleration pulse wave obtained by detecting a pulse wave generated from blood pulsation through a PPG measuring means and differentiating the detected pulse wave twice. The circulatory function diagnostic device has an advantage in that it can be used without any relevant special knowledge. However, the circulatory function diagnostic device has not yet proposed a solution against the motion artifact.
In addition, Japanese Patent Laid-Open Publication No. 1999-9564 (date of publication: Jan. 19, 1999) discloses a cardiac function diagnosing apparatus for diagnosing a cardiac function by using a heart rate obtained through various pulse wave detecting means (a photoelectric pulse wave sensor, a PPG sensor and a pressure sensor), a frequency analysis result of heart rate variability thereof, and characteristic points of the twice-differentiated PPG. The cardiac function diagnosing apparatus is formed into a glass type, a necklace type and a watch type, and also utilizes an acceleration sensor for warning the motion of the user. However, there is still a disadvantage in that it is impossible to detect the heartbeat even while the user is moving.
Furthermore, there has been published the following technical papers on the removal of motion artifacts in the PPG in the field of a pulse oximeter used for measuring a blood oxygen concentration by irradiating two specific wavelengths of light onto a finger or ear.
In the technical paper (“Noise resistance pulse oximetry using composite reference signal”, IEEE Trans. Biomed. Eng., Vol. 47, No. 8, 2000. 8) is disclosed a method of eliminating noise (i.e., motion artifact) by estimating a composite reference signal from a noise-free PPG signal (or ECG signal) and comparing the composite reference signal with the PPG signal inputted through an adaptive filter, as shown in FIG. 1. However, such a method of eliminating the noise has a limitation in that the ECG signal should ultimately be used as the reference signal since the reliability thereof is determined depending on estimation accuracy of the reference signal.
In another technical paper (“Signal processing in the pulse oximeter for eliminating the motion artifact”, 24th Spring Conference of the Korea Society of Medical and Biological Engineering, Vol. 24, No. 1, May 25 and 26, 2001) is disclosed a method of eliminating the motion artifact by passing a signal having a greatest value of correlation through a low pass filter after calculating the correlation between a signal outputted to each of respective banks and the reference signal in order to measure highly reliable oxygen concentration even under the user's motion, as shown in FIG. 2. However, considering that the motion artifact includes both high and low frequency components, the elimination of the motion artifact through the low pass filter is not very effective.